Advertisement

Advertisement

Surprisingly youthful Mars surface helps alien hunt

By Lisa Grossman in San Francisco

IF MARTIANS once existed, the Curiosity rover has a better chance of finding their traces than we ever thought.

Radioisotope dating by the rover – the first time the technique has been used beyond Earth – reveals that parts of Mars’s surface have spent much less time being fried by cosmic radiation than expected. That suggests that any microbes that once lived on Mars could still be detected, perhaps by Curiosity itself.

“This is the first attempt to do any kind of geochronology on another planet,” says Ken Farley of the California Institute of Technology, who was part of the team that presented the results at this week’s American Geophysical Union meeting in San Francisco.

This is the first time we have ever measured the age of rocks on another planet

The first surprise came in February, when Curiosity tested rocks from a depression called Yellowknife Bay and found evidence of clays laid down in the presence of non-acidic water. This water could have been home to simple microbes, but whether it was present long enough for life to get going was unclear.

On 9 December, the Curiosity team presented evidence – partly from geomorphology, partly from reanalysing the rocks – that the water was probably a lake that could have lasted for tens of thousands of years. The lake was probably part of a groundwater network that could have lasted for millions of years, giving microbes even longer to get a toehold.

Still, this doesn’t address the problem of whether any creatures that once existed left detectable traces. Mars’s warm, wet period was about 4 billion years ago, so cosmic rays would have had plenty of time to erase any fossils on the surface. But when did the surface become exposed?

Farley only thought of getting Curiosity to date the Martian surface in this way just before the rover launched&colon; he had to remotely reprogram its Sample Analysis at Mars (SAM) instrument to do it.

By comparing amounts of potassium-40 and its decay product argon-40 in the Yellowknife rocks, the team found that they were laid down about 4.2 billion years ago. But Mars’s surface was not necessarily exposed for that long.

The team then used SAM to measure the amounts of three noble-gas isotopes produced only when cosmic rays break up heavier atoms, and showed that the surface had been exposed for just 80 million years.

“That’s really young,” says rover project scientist John Grotzinger. The Mars surface was expected to be hundreds of millions – if not billions – of years old, he says.

“This really is a dramatic achievement,” says Sam Bowring of the Massachusetts Institute of Technology.

Wind eroding small cliffs called scarps probably exposed the rocks. At the end of January, the rover is due to arrive at KMS-9, a pair of scarps that look more windswept and so may have been exposed more recently – a possibility the probe can now test.

Unexpected water in the tropics

Dark streaks that grow in summer on sun-scorched slopes suggest there is water in the Martian tropics, once assumed dry. If so, it could be harder to protect them from contamination by earthly life forms, a finding that could shape future exploration.

Hints of moisture south of Mars’s equator came in 2011, when orbital images showed dark spots in late spring and summer, fading in winter. The best explanation was that ice under the surface was melting into water that seeped up and evaporated.

The latest pictures from NASA’s Mars Reconnaissance Orbiter double the number of southern sites with streaks and add some near the equator. These also appear on sunny slopes and then vanish (Nature Geoscience, doi.org/qfp). Water wasn’t expected in the tropics, which are hot enough to vaporise even buried ice, but Alfred McEwen at the University of Arizona in Tucson, who led the research, is sure that’s what it is. “As something that wets and dries again, water is a very attractive explanation.”

Spacecraft headed to Mars are sterilised before launch, but there is still a risk of contamination. A 2008 report concluded that tropical Mars is too dry for terrestrial life to survive there, making it a relatively safe choice of landing zone. The latest results could change that. Jeff Hecht

This article appeared in print under the headline “Mars’s youthful skin helps hunt for life”